Metallizable multi-ply film

ABSTRACT

A metallizable, sealable, biaxially stretch-oriented multi-ply film comprises a base layer essentially of polypropylene, a first polyolefinic facing layer and a second polyolefinic facing layer. All of the layers contain a neutralizing agent and a stabilizer against thermal degradation. The neutralizing agent is a calcium carbonate having an absolute particle size of less than 10.0 μm, a mean particle size of less than 0.1 μm, a specific surface area of more than 40 m 2  /g and a whiteness of more than 90% and, the stabilizer comprises a phenolic stabilizer having a molecular weight of more than 1,000 g/mol. One polyolefinic facing layer contains an anti-blocking agent. In addition, a process for producing the metallizable multi-ply film is described, and its use as a packaging material is indicated.

BACKGROUND OF THE INVENTION

The present invention relates to a metallizable, sealable, biaxiallystretch-oriented multi-ply film comprising a base layer essentially ofpolypropylene, a first polyolefinic facing layer and a secondpolyolefinic facing layer, all the layers containing a neutralizingagent and a stabilizer against thermal degradation.

Composite films or laminates which can be metallized are already knownfrom the state of the art. Thus, for example, EP-A 122,495 discloses afilm laminate having a surface roughness Ra from 0.2 to 1.5 μm andcomprising an oriented polypropylene film and a layer laminated thereto,which is composed of an ethylene/propylene copolymer, this layercontaining 10 to 50% by weight of the ethylene component. A metal layercan be laminated to both layers. The laminate is said to show excellentadhesive strength on metal layers, at least in comparison with paperlayers, and to exhibit good gloss and transparency. Either of the twolayers can generally contain conventional additives, such as heatstabilizers, antioxidants, antistatic agents, fillers and brighteners.

Moreover, EP-A 21,672 describes a laminate which is produced byco-extrusion and comprises an inner layer of a propylene homopolymer andat least one outer layer of an ethylene/propylene copolymer which doesnot contain any added lubricants. The laminate is corona-treated inorder to improve the adhesion of a metal layer applied to the outerlayer(s). If an outer layer is not metallized, it is sealable. Due tothe absence of lubricants from at least one of the outer, non-metallizedlayers, however, the laminate may not easily run in the machine and,owing to the composition of the outer layers, will also not show auniform corona treatment-effect promoting adhesion of the metal layer.

As shown by the discussion of the state of the art, none of thedocuments has hitherto been directed to an optimization with respect tothe sealing properties, the metal adhesion or even the metal gloss ofthe metal layer applied to the particular laminate.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved metallizable polypropylene film.

It is a particular object of the invention to provide a metallizablepolypropylene film which has been optimized with regard to thecombination of the following features:

(a) good processing properties on winding up, cutting, metallizing,imprinting or extrusion-coating or laminating, and ease of running inpackaging machines,

(b) very good and constant metal adhesion,

(c) a visually pleasing appearance pattern of the film, especially ahigh gloss of the metal layer or of the metallized surface, and

(d) good sealing properties.

It is also an object of the invention to provide a process for producingthe multi-ply film according to the invention.

Still another object of the invention resides in the provision of animproved packaging film for packaging articles.

In accomplishing the foregoing objects, there has been provided inaccordance with one aspect of the invention a metallizable, sealable,biaxially stretch-oriented multi-ply film, comprising a base layercomprised predominantly of propylene; a first polyolefinic facing layeron a first side of the base layer; a second polyolefinic facing layer ona second side of the base layer; each of the layers containing aneutralizing agent, comprising a calcium carbonate having an absoluteparticle size of less than about 10 μm, a mean particle size of lessthan about 0.1 μm, a specific surface area of more than about 40 m² /gand a whiteness of more than about 90%, and a stabilizer against thermaldegradation comprising a phenolic stabilizer having a molecular weightof more than about 1,000 g/mol; and wherein one polyolefinic facinglayer contains an anti-blocking agent. Preferably, the film alsoincludes a metal layer applied to one side of film.

In accordance with another aspect of the invention, there has beenprovided a process for producing a film as defined above, whichcomprises the steps co-extruding melts of the individual layers througha flat film die; solidifying the co-extruded film by cooling;longitudinally stretching at a temperature from about 120° to 130° C. ina ratio of from about 5 to 7:1; transversely stretching at a temperaturefrom about 160° to 170° C. in a ratio of from about 8 to 10:1; andcorona-treating one facing layer which is metallizable.

In accordance with still another aspect of the invention, there has beenprovided an improved packaging film, comprised of the metallized filmaccording to the invention, and an improved use of the packaging filmfor packaging products, such as food products.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a film of the generic type described above,wherein all the layers contain, as the neutralizing agent, a calciumcarbonate having an absolute particle size of less than about 10 μm, amean particle size of less than about 0.1 μm, a specific surface area ofmore than about 40 m² /g and a whiteness of more than about 90%. As thestabilizer, the films contain a phenolic stabilizer having a molecularweight of more than about 1,000 g/mol, and one polyolefinic facing layercontains an anti-blocking agent.

The preferred polypropylene used for the base layer within the scope ofthe present invention is an isotactic propylene homopolymer or acopolymer which is composed of propylene units as the predominantcomponent. Such polymers usually have a melting point of more than about150° C. Isotactic polypropylene having an n-heptane-soluble fraction ofless than about 15% by weight, copolymers of ethylene and propylenehaving an ethylene content of less than about 2% by weight andcopolymers of propylene with other alphaolefins having 4 to 8 carbonatoms and containing less than about 6% by weight of these alpha-olefinsare typical examples of the preferred thermoplastic polypropylene of thebase layer.

The preferred thermoplastic polymers advantageously have a melt index inthe range from about 0.5 g/10 minutes to 8 g/10 minutes at 230° C. and2.16 kg load (DIN 53 735), in particular from about 1.5 g/10 minutes to4 g/10 minutes.

The facing layer of the surface-treated side which is to be metallizedis essentially composed of an isotactic polypropylene having ann-heptane-soluble fraction of about 15% by weight or less or of apropylene/ethylene copolymer having an ethylene content of about 3% byweight or less or of a propylene/ethylene and/or butene-1 copolymerhaving an ethylene and butene content of about 5% by weight or less intotal. The melting point of the polymer for the facing layer is aboveabout 145° C.

Isotactic polypropylene is particularly preferred. The propylene polymerof this facing layer has in general a melt index from about 0.5 g/10minutes to 10 g/10 minutes, preferably from about 1.5 g/10 minutes to 4g/10 minutes, at 230° C. and 2.16 kg load (DIN 53 735).

For a further improvement of the properties, this facing layer containsa saturated hydrocarbon resin having a softening temperature from about100 to 160° C., in particular from about 120 to 140° C.

Among the numerous low-molecular resins, hydrogenated hydrocarbon resinsare preferred, especially in the form of petroleum resins, styreneresins, cyclopentadiene resins and terpene resins (these resins aredescribed in Ullmanns Enzyklopaedie der Techn. Chemie [Ullmann'sEncyclopedia of Industrial Chemistry], 4th edition, Volume 2, pages 539to 553). Hydrogenated cyclopentadiene resins have proved to beparticularly suitable hydrocarbon resins. The effective quantity ofsaturated resin is from about 2 to 12% by weight, preferably from about6 to 10% by weight, relative to the total weight of this facing layer.

The facing layer is subjected to a surface treatment by electric coronadischarge. In the corona treatment, the film surface not intended forthe surface treatment is passed over a grounded roll, above which aflame corona emanating from metallic electrical electrodes is struck.The corona discharge is usually carried out at about 10,000 V usingalternating current of about 10,000 Hz. On passing through the dischargezone, the free film surface is polarized by the high-energy chemicalprocesses taking place there and thus becomes receptive for polar mediato a certain extent.

The sealable olefinic facing layer is essentially composed of apropylene copolymer. Propylene/ethylene copolymers preferably containingfrom about 2 to 10% by weight of ethylene, propylene/butylene copolymerspreferably containing from about 0.5 to 20% by weight of butylene,ethylene/propylene/butylene terpolymers preferably containing from about0.5 to 7% by weight of ethylene and from about 5 to 30% by weight ofbutylene or mixtures of these polymers are preferred (percentages byweight relative to the total weight of the copolymer or terpolymer). Themelting point of the polymer for the sealing layers is in general fromabout 80 to 160° C., preferably from about 100 to 140° C. The melt indexof this polymer is higher than that of the base layer polymer and is ingeneral from about 1 to 12 g/10 minutes, preferably from about 3 to 9g/10 minutes, at 230° C. and 2.16 kg load (DIN 53 735).

The sealable olefinic facing layer contains conventional anti-blockingagents in a quantity from about 0.1 to 0.7% by weight, in particularfrom about 0.2 to 0.5% by weight, relative to the weight of the facinglayer. Incompatible organic polymers such as polyamide, polyester,polycarbonates and the like or inorganic substances such as silica (e.g.SYLOID 244) and/or silicates, in particular aluminum silicate (e.g. ASP900), can also be added.

All the layers, i.e. the base layer and the two facing layers, contain,as the stabilizer, a phenolic stabilizer in a quantity from about 0.05to 0.15% by weight, preferably about 0.1% by weight, relative to theparticular layer. Particularly preferably, the stabilizer content is thesame in all the layers. The stabilizer should have a molecular weight ofmore than about 1,000 g/mole. Pentaerythritoltetrakis-3-(3,5-di-tert.-butyl-4-hydroxyphenyl)-propionate (IRGANOX1010) is particularly preferred. The phenolic stabilizer can be combinedwith tris-(2,4-di-tert.-butylphenyl) phosphite.

According to the invention, all the layers likewise contain a specificcalcium carbonate as the neutralizing agent. Although calcium carbonateis frequently mentioned in general, if it is intended to eliminateadverse effects of catalyst residues on the film and film processing,for example, corrosion in the processing machines, greater importance isin general attached in this connection to additives such as calciumstearate or tripotassium citrate.

Surprisingly, it has now been found that a small quantity of thespecific calcium carbonate, incorporated into all the layers of themulti-ply film according to the invention, leads to a reduction ofcorrosion in the processing machines and, in comparison with alladditives hitherto known, does not cause any deposits and exudations,which would require production interruptions for cleaning work on theproduction machines. A risk to the quality of the films due to drippingwax drops is effectively avoided, since no exudations or deposits appearin the transverse stretching frame. Likewise, if the films are processedtogether with regenerated film arising in film production, their colorquality remains unchanged; there is no discoloration of the film ifregenerate is used.

Each of the layers contains from about 0.01 to 0.2% by weight,preferably from about 0.03 to 0.1% by weight, relative to the particularlayer, of this specific calcium carbonate, of which identicalproportions are preferably present in all the layers and the particlesize of which should be as small as possible. In absolute terms, theparticles are smaller than about 10 μm, preferably smaller than about 7μm, i.e. there should not be any particles having greater diameters, andthe mean particle diameter should be less than about 0.1 μm, preferablyless than about 0.05 μm. The specific surface area of the calciumcarbonate should be more than about 40 m² /g, preferably more than about80 m² /g. The whiteness is more than about 90%, in particular more thanabout 95%.

The thickness of the multi-ply film according to the invention can varywithin wide limits. The thickness (total thickness) of the multi-plyfilm is in general from about 10 to 40 μm, preferably from about 20 to30 μm. The facing layers are about 0.2 to 3 μm thick, preferably fromabout 0.4 to 1.5 μm and in the particularly preferred embodiment of theinvention they have the same thickness.

Via the sealable facing layer, it is possible to expand the multi-plyfilm according to the invention by bonding it to itself to give asix-ply film laminate which, if desired, can carry a metallization onboth outer layers. Likewise, it is also possible to bond single-ply,two-ply or more than three-ply laminates, in particular ofpolypropylene, via this facing layer to the multi-ply film according tothe invention.

As a result of the special design, according to the invention, of thesurface-treated facing layer, in particular the use of the specificcalcium carbonate and also the hydrogenated hydrocarbon resin, severalvery important factors, which affect the film properties, are improvedto a surprising extent.

The surface shows improved scratch resistance as compared with facinglayers modified by calcium stearate. Likewise, the surface according tothe invention shows especially an outstanding surface gloss which isparticularly effective whenever the surface is laminated with the metallayer or metallized. Precisely the gloss of the surface of the metallayer shows very clearly whether the underlying film layer issufficiently plane and smooth.

The metal layer can be composed of any suitable metal. Layers ofaluminum, zinc, gold or silver or of appropriate alloys are preferred,with aluminum or aluminum-containing alloys being particularlypreferred. Electroplating, sputtering and vacuum vapor-deposition may bementioned as suitable metallizing processes, with vacuumvapor-deposition being preferred.

The thickness of the metal layer is about 20 to 600 nm, preferably about25 to 100 nm.

If calcium stearate is used as the neutralizing agent, the adhesioneffect caused by the corona treatment diminishes more and more with timedue to migration of the reaction products of the neutralizationreaction. For this reason, the metal adhesion does not remain constant.Surprisingly, when the multi-ply film of the composition according tothe invention is used, a substantially more favorable corona decaybehavior is observed, i.e., the metal adhesion over a prolonged periodis, as a consequence thereof, substantially better than in the case ofconventional metallized laminates.

Metallized films are increasingly used for packaging purposes,especially for foodstuffs. In particular, the oxygen permeability andthe water-vapor permeability are reduced by the metal layer applied tothe plastic film, whereas the stiffness and ease of running in machinesare generally increased, and the light-proofness reaches a maximumvalue. In the journal "Neue Verpackung", No. 9, page 1048, 1st and 3rdcolumn, 1983, it is pointed out that the good barrier properties ofmetallized plastic films are persistent only if, on the one hand, theapplied metallization has excellent adhesion but, on the other hand,care is taken to avoid damage to the metal layer, especially bystretching, bending, scratching or creasing. The metallizable multi-plyfilm according to the invention is therefore also suitable for thispurpose, because of the excellent adhesion properties of the high-glossmetallizable facing layer which is planar to a particularly high degree.

The metal layer of the multi-ply film according to the invention can beprotected from mechanical damage by lamination with a polyethylene film.Thin polyethylene layers can also be applied by extrusion-coating. Themetallizable film according to the invention can be used in anoutstanding way for the production of composites with other films (oPP,PET and the like) or paper.

The metallizable multi-ply film according to the invention is preferablyproduced by co-extrusion. In this case, the melts of the individuallayers of the multi-ply film according to the invention are co-extrudedthrough a flat film die, solidified by cooling and then longitudinallystretched at a temperature from about 120 to 130° C. in a ratio of about5 to 7:1 and transversely stretched at a temperature from about 160 to170° C. in a ratio of about 8 to 10:1. Simultaneous stretching is alsopossible. Finally, the biaxially stretched film is thermofixed, and themetallizable facing layer is corona-treated. The thermofixing is carriedout at a temperature of preferably about 150 to 160° C. for a durationof about 0.5 to 10 seconds.

The incorporation of the anti-blocking agents, the stabilizers, thehydrogenated hydrocarbon resin and the specific calcium carbonate iscarried out in the known manner by means of a mixer, extruder, roll andkneader; however, a masterbatch can also be used, in particular for thespecific calcium carbonate.

The invention will be explained in more detail by reference to themerely illustrative examples which follow, without being restrictedthereto.

The multi-ply films illustrated in the examples and comparison exampleshave an overall thickness of 25 μm in each case, the facing layers eachhaving a thickness of 0.6 μm.

The three-ply films were all produced by coextrusion. This was followedby two-stage stretching with a longitudinal stretching factor of 6 and atransverse stretching factor of 9.

The metallizable facing layer was coronatreated.

EXAMPLE 1

The base layer and the metallizable facing layer were composedessentially of an isotactic propylene homopolymer having a melting pointof 165° C. The sealable facing layer was composed essentially of apropylene/ethylene copolymer containing 4.5% by weight of ethylene,relative to the copolymer. All the layers contained 0.05% by weight,relative to the particular layer, of a precipitated calcium carbonatehaving a maximum particle size of 6 μm, a mean particle diameter of 0.02μm, a specific surface area of 105 m² /g and a whiteness of 96%.Likewise, all the layers contained 0.1% by weight, relative to theparticular layer, of the IRGANOX 1010 stabilizer. In addition, themetallizable facing layer also contained 6% by weight, relative to thisfacing layer, of a fully hydrogenated cyclopentadiene resin having asoftening point of 120° C., whereas the sealable facing layer contained0.2% by weight, relative to this facing layer, of the SYLOID 244anti-blocking agent.

EXAMPLE 2

The composition of the base layer corresponded to that in Example 1. Themetallizable facing layer differed from the corresponding facing layerin Example 1 only by the polymer: instead of the isotactic propylenehomopolymer, a propylene/ethylene copolymer containing 2.5% by weight ofethylene, relative to the copolymer, and having a melting point of 150°C. was used. Instead of a propylene/ethylene copolymer, the sealablefacing layer was essentially composed of an ethylene/propylene/butyleneterpolymer containing 1.4% by weight of ethylene, 92.2% by weight ofpropylene and 6.4% by weight of butylene, relative to the terpolymer.Instead of the SYLOID 244 anti-blocking agent from Example 1, 0.3% byweight of ASP 900, relative to this facing layer, was used.

The content and type of calcium carbonate and of the stabilizer were thesame as in Example 1.

COMPARISON EXAMPLE 1

The multi-ply film of this comparison example had an ABA structure. Bothfacing layers were composed essentially of a propylene/ethylenecopolymer containing 4.5% by weight of ethylene, relative to thecopolymer. Both layers contained 0.2% by weight of SYLOID 244 as ananti-blocking agent and 0.1% by weight of IRGANOX 1010 as thestabilizer, each relative to the facing layer. Apart from the calciumcarbonate, the base layer was composed in accordance with Examples 1 and2. In place of the 0.05% by weight of calcium carbonate used in theexamples, all three layers each contained 0.05% by weight of calciumstearate as the neutralizing agent.

COMPARISON EXAMPLE 2

The example corresponded to Comparison Example 1, but with thedifference that the base layer also contained 0.2% by weight of stearicamide, relative to the base layer, in addition to the 0.05% by weight ofcalcium stearate.

The results are summarized in the Table which follows.

                  TABLE                                                           ______________________________________                                        Interfering deposits                                                          in the transverse                                                                            Scratch            Metal                                       stretching frame                                                                             resistance Gloss   adhesion                                    ______________________________________                                        Example 1   none   ++         130   ++                                        Example 2   none   ++         120   ++                                        Comparison  yes.sup.x                                                                            --          90   +-                                        Example 1                                                                     Comparison  yes.sup.x                                                                            --          90   --                                        Example 2                                                                     ______________________________________                                         ++ = good, +- = moderate, -- = poor                                      

(x) In all the comparison examples, interfering deposits of stearic acidoccur in the transverse stretching frame, which soil the frame or driponto the film web and, as an interfering wax drop, impair the quality ofthe film.

The gloss was determined in accordance with DIN 67 530, using an RBreflectometer from Dr. Lange (Federal Republic of Germany). The gloss istermed good if it is above 110.

The scratch resistance of a film is understood as the resistance of afilm surface to changes due to mechanical stress, in particular africtional stress. As a rule, the rise in turbidity ΔT, caused by thescratching, is used as a measure of the scratch resistance of a film.

    ΔT=Tv-To

Tv is here to be understood as the turbidity of the scratched film, andTo is to be understood as the turbidity of the unchanged original film,the turbidity of one film ply being determined in each case during thescratch resistance measurement.

The scratch resistance against frictional stress was determined inaccordance with DIN 53 754. The measurements were carried out using anabrasion measuring apparatus of 1161 type from Frank (Federal Republicof Germany), friction wheels of the CALIBRASE CS-10F brand from Teledyne(USA) being used and loaded with 250 g.

The scratch resistance is termed good if the rise in turbidity is lessthan 20%.

The metal adhesion is assessed by means of an adhesive tape test. Anadhesive tape is stuck to the metal layer and slowly peeled off. Themetal adhesion is termed good if 0% of metal is peeled off. The metaladhesion is termed moderate if less than 10% is peeled off. In the caseof more than 10% of metal being peeled off, the metal adhesion is termedpoor.

What is claimed is:
 1. A metallizable, sealable, biaxiallystretch-oriented multi-ply film, consisting essentially ofa base layercomprised predominantly of propylene; a first polyolefinic facing layeron a first side of said base layer; a second polyolefinic facing layeron a second side of said base layer; each of said layers containing aneutralizing agent, comprising a calcium carbonate having an absoluteparticle size of less than about 10 μm, a mean particle size of lessthan about 0.1 μm, a specific surface area of more than about 40 m² /gand a whiteness of more than about 90%, in a quantity of from about0.01% to 0.2% by weight, relative to the weight of the layer, and astabilizer against thermal degradation comprising a phenolic stabilizerhaving a molecular weight of more than about 1000 g/mol; and wherein onepolyolefinic facing layer contains an anti-blocking agent.
 2. A film asclaimed in claim 1, wherein at least one polyolefinic facing layer isheat-sealable.
 3. A film as claimed in claim 2, wherein theheat-sealable facing layer contains an anti-blocking agent.
 4. A film asclaimed in claim 1, wherein one polyolefinic facing layer isnon-heat-sealable.
 5. A film as claimed in claim 4, wherein thenon-sealable facing layer has been corona-treated.
 6. A film as claimedin claim 4, wherein the non-sealable facing layer contains a saturatedhydrocarbon resin.
 7. A film as claimed in claim 6, wherein thehydrocarbon resin comprises a hydrogenated cyclopentadiene resin whichis present in a quantity from about 2% to 12% by weight, relative to theweight of the facing layer.
 8. A film as claimed in claim 2, wherein thesealable layer contains the anti-blocking agent in a quantity from about0.1% to 0.7% by weight, relative to the weight of this layer.
 9. A filmas claimed in claim 8, wherein the anti-blocking agent contains silicaor aluminum silicate.
 10. A film as claimed in claim 1, which has anoverall thickness from about 10 to 40 μm, the facing layers each beingfrom about 0.2 to 0.7 μm thick.
 11. A film as claimed in claim 5,further comprising a metal layer metallized or laminated to the outsideof the non-sealable, surface-treated facing layer.
 12. A film as claimedin claim 11, wherein the metal layer comprises aluminum applied byvacuum vapor-deposition.
 13. A film as claimed in claim 11, wherein themetal layer comprises an aluminum foil.
 14. A film as claimed in claim11, wherein the metal layer is bonded to the non-sealable,surface-treated layer by means of a solvent-free, two-componentadhesive.
 15. A film comprising a film as claimed in claim 11, furthercomprising a protective coating on the outside of the metal layer.
 16. Afilm as claimed in claim 15, wherein the protective coating comprises apolyolefinic protective coating.
 17. A film as claimed in claim 16,wherein the protective coating comprises a polyethylene film or apolypropylene film or a polyester film.
 18. A film as claimed in claim15, which is doubled on itself via its sealable facing layer or isbonded via this facing layer to a single-ply or two-ply polypropylenefilm.
 19. A packaging film, especially in a film composite or papercomposite, comprising a multi-ply film as defined by claim 1.